The present invention generally relates to call recovery in a mobile communications system.
Handoff is a general term describing the function of transferring a communication signal for a mobile subscriber from one base station transceiver to another base station transceiver as a subscriber negotiates the wireless territory. There are two general methods of conducting handoff in a wireless communication system, soft and hard handoff. Quality may suffer using either handoff method as the mobile subscriber switches between base stations for individual cells.
Hard handoff typically occurs near the boundaries between cells. Through continuous measurements of received signal power from a mobile subscriber, the base station where the subscriber has established communication determines if the power is reduced below a nominal value near the cell boundary. The hard handoff to a candidate cell's base station occurs instantaneously without disrupting the call in progress. At handoff, the received power at the candidate cell's base station is much greater than required to ensure an ideal handoff. The difference between the current cell's power and the candidate cell's power; however, significantly reduces system capacity by interfering with other users.
A soft handoff occurs throughout a given range of distances from the current and candidate cells' base stations. In the soft handoff method, a mobile subscriber is connected to both base stations as the subscriber travels near the common cell boundary. The decision to switch is made depending upon the reception of the mobile subscriber's pilot signal. A central switching center, often referred to as a mobile switching center, decides at what point one of the base stations should be dropped. Shared communication is performed for a finite period of time, during which time transmission from the current and candidate cells is required.
In a typical wireless communication system, the plurality of signals are transmitted within the same frequency band. This not only applies to mobile subscribers in the same cell, but also to those in all other cells. Since the same frequencies are used, the transmitted power levels from the mobile subscriber and the base station must be monitored closely. If power control is not strictly adhered to, the overall transmission interference and the total number of usable channels is adversely affected. Therefore, the number of signals which can be successfully transmitted and received is associated with the total power of all users.
Both soft and hard handoff methods have shortcomings. Hard handoff can potentially suffer from high drop-out rates. The soft handoff method requires a duplication of transmission resources from the current base station and a base station from at least one candidate cell. The mobile subscriber must establish two concurrent communication links, thereby requiring twice the transmission power which would otherwise be required. The increase in transmission power wastes power, capacity of the air interface to carry transmissions, and contributes to total system interference while the handoff is taking place. The power or energy transmitted in a wireless system by each mobile subscriber should be kept at the minimum necessary to convey information and to minimize interference with the other users. Careful control of transmission power also contributes to extended use of portable devices relying on battery power. Furthermore, diversity combining of the duplicate signals at the central switching center is cumbersome unless the delay from both cell base stations is nearly identical.